The present invention relates to a semiconductor laser device, and more particularly to a semiconductor laser device whose laser element has an enhanced reliability.
It is well known that a semiconductor laser is very susceptible to high temperatures brought about by the generation of heat by the semiconductor laser itself and to changes in ambient temperature, and that these factors can lead to a lowering of the optical power and a degradation in the device lifetime.
In order to solve these problems concerning temperature, a prior-art semiconductor laser device has been furnished with a heat sink (also called "submount") of good thermal conduction and/or a photosensor for monitoring the optical power, or the like. The heat sink or submount is usually made of copper. Since, however, copper is inferior in workability, silicon (Si) is well known as a substitute material from, e.g., the official gazette of Japanese Utility Model Registration Application Publication No. 54-19829. As is known from the official gazette of Japanese Utility Model Registration Application Publication No. 54-3660, utilizing a silicon chip as the submount makes it possible to form the photosensor within the submount. The number of components required to constitute the semiconductor laser device can therefore be reduced, which permits the advantages of a lower cost and a smaller size.
The aforementioned official gazette of Japanese Utility Model Registration Application Publication No. 54-3660 discloses a technique wherein the laser radiation of the semiconductor laser is received by the photosensor, and current to flow through the semiconductor laser is adjusted according to the variation of the ambient temperature, whereby the optical power of the semiconductor laser is controlled to be constant. That is, the official gazette discloses the semiconductor laser device furnished with automatic power control means. Such automatic power control means operates very effectively when the ambient temperature is near 25.degree. C., but it is functionally limited when the ambient temperature is too high or too low. Therefore, a prescribed optical power becomes unattainable even with the automatic power control means.
As seen from the official gazette of Japanese Laid-open Patent Application No. 50-81695, there has been proposed a semiconductor laser device including, besides the photosensor, a theremoelectric heat pump which executes cooling by utilizing the Peltier effect. This semiconductor laser device disclosed in the official gazette of Japanese Laid-open Patent Application No. 50-81695 can very advantageously increase the lifetime of a semiconductor laser element in such a way that the thermoelectric heat pump is used for holding the temperature of the semiconductor laser element constant, thereby to keep the optical power constant.
With such semiconductor laser device, however, a submount is made of copper, and the photosensor is formed in a substrate which is made of a material different from that of the submount. Accordingly, the number of components which constitutes the semiconductor laser device becomes large. This results in the problem that the device cannot be made low in cost and small in size. Moreover, this device is based on automatic power control means for monitoring laser radiation with the photosensor, as is the case with to the aforementioned semiconductor laser device disclosed in the official gazette of Japanese Utility Model Registration Application Publication No. 54-3660. It is therefore limited in attaining a prescribed optical power against ambient temperatures.